There is growing evidence showing the various health benefits of the life-saving Vitamin D. If your vitamin D level is sub-optimal, your doctor may write you a prescription for vitamin D to bring the blood level up. If you or someone you know is taking vitamin D from prescription, please read on.

Many "vitamin D" supplements contain vitamin D2, which is obtained by subjecting ergosterol from yeast, a chemical found in plants, to radiation.

Recently, news reports declared that mushrooms subjected to ultraviolet radiation are "[zapped] into a giant serving" of vitamin D.7 Like plants, any vitamin D contained in mushrooms is in the form of vitamin D2.8

Should sources of vitamin D2 really be considered sources of vitamin D?

Some researchers claim that vitamin D2, also called "ergocalciferol" and called "viosterol" in the old days, and vitamin D3, also called "cholecalciferol" are equally effective in humans because their ability to bind to the vitamin D receptors in our cells is equal.2 But that's not the whole story.

Figure 3: The Chemical Structure of Vitamin D3 (Cholecalciferol)

Figure 4: The Chemical Structure of Vitamin D2 (Ergosterol)

Vitamin D is carried in the blood by vitamin D-binding protein (DBP). DBP is kind of like a savings account for vitamin D. If you didn't have the DBP, you'd be forced to use all your vitamin D as soon as you absorb it, and excrete the rest. This would be a giant waste of vitamin D, because you can only use so much at a time. DBP thus helps to increase the effect of a given dose of vitamin D by holding on to what you don't need at any given moment for later use, and helps prevent toxicity by keeping the portion you don't need at any given moment from being delivered to your cells.2

Although vitamin D2 binds well to the vitamin D receptor, it has very little affinity for vitamin D-binding protein. For this reason, it is well-known to be useless in chickens and other birds. When vitamin D was seen merely as a cure for rickets, vitamin D2's ability to treat rickets in the small amounts needed led researchers to believe it equal in power to vitamin D3 in humans. Now that researchers are uncovering the need for much higher levels of vitamin D to maintain optimal health, it is becoming clear that vitamin D2 just doesn't fit the bill.

The researchers Laura Armas, Bruce Hollis, and Robert Heaney showed in 2004 that vitamin D2's low affinity for the vitamin D-binding protein makes it nearly ten times less effective at raising long-term vitamin D levels.9

If vitamin D2 has a lower affinity for the DBP, it follows that it is also much more likely to result in toxicity than is vitamin D3. It is therefore unsurprising that, according to Dr. John Cannel, president of the Vitamin D Council, nearly all cases of toxicity from pharmacological doses of vitamin D resulted from the consumption of vitamin D2.10

The vitamin D2 synthesized from plant sterols should therefore not be considered true vitamin D for humans. Humans should obtain vitamin D from the sun and from the vitamin D-rich fatty animal foods that provide the form of vitamin D with which the sun provides us, and which we have consumed throughout our evolution.